Projection lens and projection display device using the same

ABSTRACT

Disclosed are a projection lens that has a small size, a simple inner focus structure, a high optical performance, and a wide angle of view of about 75 degrees or more, and a projection display device. 
     A projection lens includes a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power arranged in this order from a magnification side. A fifth lens arranged closest to a reduction side in the first lens group is moved along an optical axis to adjust focus. The projection lens satisfies the following conditional expression:
 
3.0&lt; D   G12   /f &lt;6.0
         where D G12  indicates a distance between the first lens group and the second lens group and f indicates the focal length of the entire lens system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2008-218202 filed on Aug. 27, 2008; theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection lens that enlarges andprojects, for example, display information from a light valve, such as atransmissive or reflective liquid crystal display device or a DMD(digital micro-mirror device), and more particularly, to a projectionlens applicable for a so-called front projection display device, and aprojection display device using the same.

2. Description of the Related Art

In recent years, projection display devices using light valves, such asa liquid crystal display device or a DMD display device, have come intowidespread use. In particular, a projection display device has beenwidely used which uses three light valves corresponding to illuminationlight components of three primary colors, such as R, G, and B, tomodulate the illumination light components, combines the lightcomponents modulated by the three light valves using, for example, aprism, and displays an image on a screen using a projection lens.

In the projection display device that uses a color composition opticalsystem to compose the modulated light components from the three lightvalves and projects the composed light, the projection lens needs tohave a large back focal length in order to arrange a prism for colorcomposition as a characteristic of the projection lens. In addition, thespectral characteristics of the color composition optical system varydepending on the angle of incident light. Therefore, the projection lensneeds to have characteristics that an entrance pupil is positioned at asufficiently long distance as viewed from the reduction side. That is,the projection lens needs to have telecentricity. The projection lensalso needs to correct aberration according to the resolution of adevice.

As the size of the light valve is reduced, the precision thereof isimproved, and personal computers are widely spread, the use of theprojection display device for presentation is increased. Therefore,there is an increasing demand for a projection display device having ahigh performance, high brightness, and a small size.

However, as the performance and brightness of the projection lens areincreased, the size of the lens is increased, and the size of a movingmechanism for moving a lens group to adjust focus is also increased.

An inner focus type in which only some lenses in a lens group can bemoved to easily adjust focus has been known in the field of an imaginglens of a single-lens reflex camera or video camera. However, when thisinner focus type is applied to a projection lens without any change, anemission angle is small, and it is difficult to ensure telecentricity.

Therefore, in the projection display device using the inner focus lens,when a color composition prism is used to perform color composition,color unevenness occurs on a screen. Therefore, it is difficult to usethe inner focus lens according to the related art as a projection lensof a projector, without changing the structure of the inner focus lens.

Therefore, in order to solve the above-mentioned problems,JP-A-2001-166205 discloses a projection lens and a projector apparatus.

That is, the projection lens disclosed in JP-A-2001-166205 includes afirst negative lens group, a second positive lens group, and a thirdpositive lens group arranged in this order from a magnification side,and a lens arranged closest to a reduction side in the first lens groupis moved along the optical axis X to adjust focus. In addition, the gapbetween the first lens group and the second lens group, and the focallength of the entire lens system are set in a predetermined range.

The projection lens having the above-mentioned structure has anappropriate back focal length, telecentricity, a high opticalperformance capable of reducing distortion and chromatic aberration, asimple inner focus structure, and a small size.

However, in the related art disclosed in JP-A-2001-166205, a total angleof view is about 69 degrees. In recent years, a wide-angle projectionlens having a high optical performance and a total angle of view ofabout 75 degrees or more has been required in order to meet a demand fora projection display device that can be used in a small space in which adistance to the screen is small. However, it is difficult for therelated art disclosed in JP-A-2001-166205 to meet the demand.

SUMMARY OF THE INVENTION

The invention has been made in order to solve the above-mentionedproblems, and an object of the invention is to provide a projection lenshaving a simple inner focus structure, an appropriate back focal length,telecentricity, a high optical performance, a wide angle of view ofabout 75 degrees or more, and a small size, and a projection displaydevice using the same.

According to an aspect of the invention, a projection lens includes: afirst lens group having a negative refractive power; a second lens grouphaving a positive refractive power; and a third lens group having apositive refractive power. The first to third lens groups are arrangedin this order from a magnification side. A lens unit arranged closest toa reduction side in the first lens group is moved along an optical axisto adjust focus. The projection lens satisfies Conditional expression 1given blow:3.0<D _(G12) /f<6.0  [Conditional expression 1]

(where D_(G12) indicates a distance between the first lens group and thesecond lens group and f indicates the focal length of the entire lenssystem).

Here, examples of the term ‘lens unit’ include a single lens and acemented lens.

The projection lens may satisfy Conditional expression 2 given below:−8.0<f ₁ /f<−3.0  [Conditional expression 2]

(where f₁ indicates the focal length of the first lens group).

The projection lens may satisfy Conditional expression 3 given below:2.5<f ₂₃ /f<6.0  [Conditional expression 3]

(where f₂₃ indicates a composite focal length of the second lens groupand the third lens group).

In the projection lens according to the above-mentioned aspect, the lensunit arranged closest to the reduction side in the first lens group maybe a single positive lens having a convex surface facing the reductionside.

The projection lens may satisfy Conditional expression 4 given below:1.8<n _(1G)  [Conditional expression 4]

(where n_(1G) indicates the average value of the refractive indexes oflenses in the first lens group with respect to the d-line, except forthe lens unit arranged closest to the reduction side in the first lensgroup).

In the projection lens according to the above-mentioned aspect, a lensarranged closest to the reduction side in the third lens group may be acemented lens of a positive lens and a negative lens.

According to another aspect of the invention, a projection displaydevice includes: at least one light source; at least one light valve; atleast one illumination optical unit that guides light emitted from thelight source to the light valve; and the projection lens according tothe above-mentioned aspect. The light valve modulates the light emittedfrom the light source, and the modulated light is projected onto ascreen by the projection lens.

According to the projection lens and the projection display device ofthe above-mentioned aspects of the invention, a lens arranged closest tothe reduction side in the first lens group is moved along the opticalaxis X to adjust focus. When the gap between the first lens group andthe second lens group is D_(G12) and the focal length of the entire lenssystem is f, the projection lens satisfies Conditional expression 1:3.0<D_(G12)/f<6.0.

In this way, it is possible to obtain such a large back focal lengththat a color composition optical system can be inserted, and ensuretelecentricity. Therefore, it is possible to reduce distortion andchromatic aberration and obtain a high optical performance.

When Conditional expression 1 is satisfied, it is possible to obtain aprojection lens having a high optical performance and a wide total angleof view of about 75 degrees or more, as compared to the related artdisclosed in JP-A-2001-166205. In addition, it is possible to reduce thesize of the entire lens system.

The projection display device according to the above-mentioned aspect ofthe invention uses the projection lens according to the above-mentionedaspect. Therefore, the projection display device can have a wide totalangle of view of 75 degrees or more and a high optical performance. Inaddition, it is possible to reduce the size of the entire lens system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the structure of a projection lensaccording to Example 1 of the invention;

FIG. 2 is a diagram illustrating the structure of a projection lensaccording to Example 2 of the invention;

FIG. 3 is a diagram illustrating the structure of a projection lensaccording to Example 3 of the invention;

FIG. 4 is a diagram illustrating all aberrations of the projection lensaccording to Example 1, where (A) shows spherical aberration, (B) showsastigmatism, (C) shows distortion, and (D) shows lateral chromaticaberration;

FIG. 5 is a diagram illustrating all aberrations of the projection lensaccording to Example 2, where (A) shows spherical aberration, (B) showsastigmatism, (C) shows distortion, and (D) shows lateral chromaticaberration;

FIG. 6 is a diagram illustrating all aberrations of the projection lensaccording to Example 3, where (A) shows spherical aberration, (B) showsastigmatism, (C) shows distortion, and (D) shows lateral chromaticaberration; and

FIG. 7 is a diagram schematically illustrating the structure of a mainpart of a projection display device according to an embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, exemplary embodiments of the invention will be describedwith reference to the accompanying drawings. FIG. 1 is a diagramillustrating a projection lens according to an embodiment of theinvention, and shows the structure of a lens according to Example 1,which will be described below. This lens will be described below as arepresentative example of this embodiment. In FIG. 1, X indicates anoptical axis.

The projection lens according to this embodiment is a fixed-focus lens,and includes a first lens group G₁ having a negative refractive power, asecond lens group G₂ having a positive refractive power, and a thirdlens group G₃ having a positive refractive power arranged in this orderfrom a magnification side. In addition, the reduction side of theprojection lens is substantially telecentric.

The first lens group G₁ includes five lenses, that is, positive,negative, negative, negative, and positive lenses arranged in this orderfrom the magnification side. In addition, the second lens group G₂includes two negative and positive lenses. The third lens group G₃includes seven lenses, that is, positive, negative, positive, negative,positive, positive, and negative lenses arranged in this order from themagnification side (in Example 2, six lenses, that is, negative,positive, negative, positive, positive, and negative arranged in thisorder from the magnification side, and in Example 3, six lenses, thatis, positive, negative, positive, negative, positive, and negativelenses arranged in this order from the magnification side).

An aperture diaphragm 3 is provided between the second lens group G₂ andthe third lens group G₃ (a mask may also be provided at a differentposition).

In the projection lens shown in FIG. 1, a light beam that is incidentfrom the right side of FIG. 1 and is given image information from animage display surface 1 of a light valve is incident on the projectionlens through a glass block (which includes various filters, such as alow pass filter or an infrared cut filter) 2, and is enlarged andprojected to the left side of FIG. 1 by the projection lens. FIG. 1shows only one image display surface 1 for easy viewing. However, in aprojection display device, a color separation optical system separates alight beam emitted from a light source into three primary color lightbeams, and light valves are provided for the three primary color lightbeams to display a full color image.

Specifically, a color composition unit (glass block), such as a crossdichroic prism, may be provided at the position of the glass block 2 tocompose the three primary color light beams.

It is preferable to move a lens unit (a fifth lens L₅ in FIG. 1)arranged closest to the reduction side in the first lens group G₁ in theoptical axis direction to adjust focus.

When the lens unit arranged closest to the reduction side in the firstlens group G₁ is used as a focus group to adjust focus, it is possibleto make the total length of the lens system constant.

The projection lens according to this embodiment satisfies Conditionalexpressions 1 to 4 given below:3.0<D _(G12) /f<6.0,  [Conditional expression 1]−8.0<f ₁ /f<−3.0,  [Conditional expression 2]2.5<f ₂₃ /f<6.0, and  [Conditional expression 3]1.8<n _(1G)  [Conditional expression 4]

(where D_(G12) indicates the distance between the first lens group G₁and the second lens group G₂, f indicates the focal length of the entirelens system, f₁ indicates the focal length of the first lens group G₁,f₂₃ indicates a composite focal length of the second lens group G₂ andthe third lens group G₃, and n_(1G) indicates the average value of therefractive indexes of lenses in the first lens group G₁ with respect tothe d-line, except for the lens unit arranged closest to the reductionside in the first lens group G₁).

According to the above-mentioned structure, the projection lensaccording to this embodiment can have an appropriate back focal length,a wide angle of view, a high optical performance, and a small size.

Hereinafter, the meaning of Conditional expressions 1 to 4 will bedescribed.

First, if the ratio is greater than the upper limit of Conditionalexpression 1, it is difficult to reduce the size of the entire lenssystem. On the other hand, if the ratio is less than the lower limit ofConditional expression 1, it is difficult to ensure a total angle ofview of about 75 degrees or more. Therefore, it is difficult to respondto a demand for increasing an angle of view.

Conditional expression 1 and the Conditional expression 2 are set so asto be established at all focus positions.

In addition, the projection lens may be configured so as to satisfy thefollowing Conditional expression 1′ instead of Conditional expression 1:3.5<D _(G12) /f<5.5.  [Conditional expression 1′]

In this case, it is possible to further increase an angle of view whilereducing the size of the entire lens system.

If the values of f₁/f and f₂₃/f are greater than the upper limits ofConditional expressions 2 and 3 or less than the lower limits thereof,the power balance between the first lens group G₁ and the second andthird lens groups G₂ and G₃ is broken, and it is difficult to obtain anappropriate back focal length or correct all aberrations. Therefore, theprojection lens is configured so as to satisfy Conditional expressions 2and 3. In this case, the power balance between the first lens group G₁and the second and third lens groups G₂ and G₃ is maintained, and anappropriate back focal length is obtained. As a result, it is easy tocorrect all aberrations.

If the average value is less than the lower limit of Conditionalexpression 4, it is difficult to increase an angle of view. Therefore,in this embodiment, the projection lens is configured so as to satisfyConditional expression 4. In this case, it is possible to obtain a totalangle of view of about 75 degrees or more.

The projection lens disclosed in JP-A-2001-166205 has a small totalangle of view of about 69 degrees or less. However, as described above,the projection lens according to this embodiment is configured such thatthe value of D_(G12)/f is in the range of 3.0 to 6.0. Therefore, it ispossible to ensure a total angle of view of about 75 degrees or more.

Next, a projection display device according to an embodiment of theinvention will be described. FIG. 7 is a diagram illustrating an exampleof the structure of a main part (illumination optical system 10) of theprojection display device according to the embodiment of the invention.

As shown in FIG. 7, the illumination optical system 10 includestransmissive liquid crystal panels 11 a to 11 c, serving as lightvalves, dichroic mirrors 12 and 13 for color separation, a crossdichroic prism 14 for color composition, condenser lenses 16 a to 16 c,and total reflecting mirrors 18 a to 18 c. The previous stage of thedichroic mirror 12 is not shown in FIG. 7. White light emitted from thelight source is incident on the liquid crystal panels 11 a to 11 ccorresponding to three color light beams (G light, B light, and R light)through the illumination optical unit and then modulated. Then, themodulated light beams are projected onto a screen by the projectionlens.

Since the projection display device uses the projection lens accordingthe embodiment of the invention, it is possible to effectively correctchromatic aberration and obtain a large high-resolution screen.

Next, detailed examples of the projection lens according to theembodiment of the invention will be described. In the followingexamples, members having the same operation and effect are denoted bythe same reference numerals.

Example 1

As shown in FIG. 1, a projection lens according to Example 1 includes afirst lens group G₁ having a negative refractive power, a second lensgroup G₂ having a positive refractive power, and a third lens group G₃having a positive refractive power arranged in this order from themagnification side, and the reduction side of the projection lens issubstantially telecentric. The first lens group G₁ includes five lenses,that is, a first lens L₁, which is a positive meniscus lens having aconvex surface facing the magnification side, second and third lenses L₂and L₃, which are negative meniscus lenses each having a convex surfacefacing the magnification side, a fourth lens L₄, which is a biconcavelens, and a fifth lens L₅, which is a positive meniscus lens having aconvex surface facing the reduction side, arranged in this order fromthe magnification side.

The second lens group G₂ includes a cemented lens of a sixth lens L₆,which is a negative meniscus lens having a convex surface facing themagnification side, and a seventh lens L₇, which is a plano-convex lenshaving a convex surface facing the magnification side.

The third lens group G₃ includes seven lenses, that is, a cemented lensof an eighth lens L₈, which is a biconvex lens, and a ninth lens L₉,which is a biconcave lens, a cemented lens of a tenth lens L₁₀, which isa biconvex lens, and an eleventh lens L₁₁, which is a negative meniscuslens having a convex surface facing the reduction side, a twelfth lensL₁₂, which is a biconvex lens, and a cemented lens of a thirteenth lensL₁₃, which is a biconvex lens, and a fourteenth lens L₁₄, which is anegative meniscus lens having a convex surface facing the reductionside.

An aperture diaphragm (or a mask, which is the same with the followingexamples) 3 is provided between the second lens group G₂ and the thirdlens group G₃.

The fifth lens L₅ arranged closest to the reduction side in the firstlens group G₁ is moved in the optical axis direction to adjust focus.The fifth lens L₅ is a single positive spherical lens having a convexsurface facing the reduction side, and has a certain degree of power. Inthis way, it is possible to reduce the weight and the movement distanceof a focus group. As a result, it is possible to more effectively usethe first lens group as a focus group.

The projection lens according to Example 1 is configured so as tosatisfy Conditional expressions 1 to 4 (and Conditional expression 1′).

FIG. 1 also shows the image display surface 1 of the light valve, theglass block 2, and the aperture diaphragm 3.

In Table 1, an upper part shows the curvature radius R of each lenssurface in the projection lens according to Example 1 (the focal lengthof the entire lens system is normalized to 1.00, which is similarlyapplied to the following examples), the thickness of the center of eachlens and an air space (hereinafter, referred to as an on-axis surfacespacing) D between the lenses (the focal length of the entire lenssystem is normalized to 1.00, which is similarly applied to thefollowing examples), and the refractive index N_(d) and the Abbe numberν_(d) of each lens with respect to the d-line. In Table 1 and Tables 2to 4, which will be described below, a surface number is sequentiallyincreased from the magnification side.

In Table 1, a lower part shows a variable spacing 1 (the gap between thefourth lens L₄ and the fifth lens L₅) and a variable spacing 2 (the gapbetween the fifth lens L₅ and the sixth lens L₆) when a magnificationside power is 126.1 and 430.3. The term ‘magnification side power’ doesnot mean power that varies when a projection distance is constant, butmeans power that varies depending on the projection distance in thefocusing range.

TABLE 1 Surface number R D Nd νd  1 5.503 0.959 1.8348 42.7  2 13.5650.015  3 3.955 0.224 1.8348 42.7  4 2.499 0.803  5 9.775 0.167 1.834842.7  6 2.663 0.983  7 −4.230 0.140 1.8466 23.9  8 5.453 Variablespacing 1  9 −18.366 0.646 1.7550 52.3 10 −3.770 Variable spacing 2 116.920 0.129 1.8040 46.6 12 2.268 0.561 1.6727 32.1 13 ∞ 2.499 14 ∞ 2.091(Mask) 15 3.741 0.436 1.8340 37.2 16 −3.149 0.129 1.6889 31.1 17 2.1830.146 18 6.815 0.525 1.4875 70.4 19 −1.673 0.129 1.8340 37.2 20 −4.8700.015 21 18.887 0.322 1.6204 60.3 22 −9.193 0.015 23 7.067 0.627 1.618063.3 24 −2.588 0.167 1.8466 23.9 25 −3.437 0.908 26 ∞ 2.430 1.5163 64.027 ∞ Magnification side power 126.1 times 430.3 times Variable spacing 12.322 2.305 Variable spacing 2 4.001 4.018

Values corresponding to Conditional expressions 1 to 4 (and Conditionalexpression 1′) in Example 1 are shown in Table 4, which will bedescribed below. As can be seen from Table 4, Example 1 satisfies all ofConditional expressions 1 to 4 (and Conditional expression 1′).

Example 2

FIG. 2 shows the structure of a projection lens according to Example 2.The projection lens according to Example 2 is similar to that accordingto Example 1 in that it includes a first lens group G₁ having a negativerefractive power, a second lens group G₂ having a positive refractivepower, and a third lens group G₃ having a positive refractive powerarranged in this order from the magnification side and the reductionside thereof is substantially telecentric, but is different therefrom inthe structure of the third lens group G₃.

That is, the third lens group G₃ includes six lenses, that is, an eighthlens L₈, which is a negative meniscus lens having a convex surfacefacing the magnification side, a cemented lens of a ninth lens L₉, whichis a biconvex lens, and a tenth lens L₁₀, which is a negative meniscuslens having a convex surface facing the reduction side, an eleventh lensL₁₁, which is a biconvex lens, and a cemented lens of a twelfth lensL₁₂, which is a biconvex lens, and a thirteenth lens L₁₃, which is anegative meniscus lens having a convex surface facing the reductionside, arranged in this order from the magnification side. In addition,the projection lens according to Example 2 is similar to that accordingto Example 1 in the lens structure of the first lens group G₁ and thesecond lens group G₂ and in that the fifth lens L₅ is moved in theoptical axis direction to adjust focus.

In Table 2, an upper part shows the curvature radius R of each lenssurface in the projection lens according to Example 2, the on-axissurface spacing D between the lenses, and the refractive index N_(d) andthe Abbe number ν_(d) of each lens with respect to the d-line. In Table2, a lower part shows the variable spacing 1 (the gap between the fourthlens L₄ and the fifth lens L₅) and the variable spacing 2 (the gapbetween the fifth lens L₅ and the sixth lens L₆) when a magnificationside power is 126.1 and 430.3.

TABLE 2 Surface number R D Nd νd  1 5.495 1.056 1.8348 42.7  2 13.3080.015  3 4.185 0.380 1.8348 42.7  4 2.312 0.778  5 9.601 0.167 1.834842.7  6 2.597 0.866  7 −4.569 0.221 1.8466 23.9  8 5.822 Variablespacing 1  9 −12.574 0.568 1.6968 55.5 10 −3.674 Variable spacing 2 114.967 0.129 1.8010 35.0 12 2.221 0.537 1.6990 30.1 13 ∞ 1.554 14 ∞ 2.091(Mask) 15 4.048 0.336 1.8052 25.4 16 2.421 0.141 17 8.363 0.490 1.518258.9 18 −1.600 0.216 1.8052 25.4 19 −4.471 0.015 20 24.414 0.322 1.658450.9 21 −9.455 0.018 22 7.155 0.705 1.6180 63.3 23 −2.523 0.228 1.846623.9 24 −3.249 0.152 25 ∞ 3.184 1.5163 64.0 26 ∞ Magnification sidepower 126.1 times 430.3 times Variable spacing 1 2.222 2.205 Variablespacing 2 5.010 5.028

Values corresponding to Conditional expressions 1 to 4 (and Conditionalexpression 1′) in Example 2 are shown in Table 4, which will bedescribed below. As can be seen from Table 4, Example 2 satisfies all ofConditional expressions 1 to 4 (and Conditional expression 1′).

Example 3

FIG. 3 shows the structure of a projection lens according to Example 3.The projection lens according to Example 3 is similar to that accordingto Example 1 in that it includes a first lens group G₁ having a negativerefractive power, a second lens group G₂ having a positive refractivepower, and a third lens group G₃ having a positive refractive powerarranged in this order from the magnification side and the reductionside thereof is substantially telecentric, but is different therefrom inthe structure of the third lens group G₃.

That is, the third lens group G₃ includes six lenses, that is, acemented lens of an eighth lens L₈, which is a biconvex lens, and aninth lens L₉, which is a biconcave lens, a cemented lens of a tenthlens L₁₀, which is a biconvex lens, and an eleventh lens L₁₁, which is anegative meniscus lens having a convex surface facing the reductionside, and a cemented lens of a twelfth lens L₁₂, which is a biconvexlens, and a thirteenth lens L₁₃, which is a negative meniscus lenshaving a convex surface facing the reduction side, arranged in thisorder from the magnification side.

In addition, the projection lens according to Example 3 is substantiallysimilar to that according to Example 1 in the lens structure of thefirst lens group G₁ and the second lens group G₂ and in that the fifthlens L₅ is moved in the optical axis direction to adjust focus.

In Table 3, an upper part shows the curvature radius R of each lenssurface in the projection lens according to Example 3, the on-axissurface spacing D between the lenses, and the refractive index N_(d) andthe Abbe number ν_(d) of each lens with respect to the d-line. In Table3, a lower part shows the variable spacing 1 (the gap between the fourthlens L₄ and the fifth lens L₅) and the variable spacing 2 (the gapbetween the fifth lens L₅ and the sixth lens L₆) when a magnificationside power is 126.1 and 430.3.

TABLE 3 Surface number R D Nd νd  1 5.611 0.959 1.8348 42.7  2 13.5980.015  3 4.181 0.299 1.8348 42.7  4 2.447 0.816  5 9.829 0.167 1.834842.7  6 2.721 0.932  7 −4.704 0.266 1.8466 23.9  8 6.202 Variablespacing 1  9 −15.825 0.606 1.7725 49.6 10 −4.015 Variable spacing 2 116.223 0.179 1.7995 42.2 12 2.231 0.524 1.6727 32.1 13 ∞ 1.908 14 ∞ 2.091(Mask) 15 3.843 0.429 1.8010 35.0 16 −3.092 0.129 1.6398 34.5 17 2.1350.135 18 5.734 0.526 1.5163 64.1 19 −1.619 0.228 1.8052 25.4 20 −4.8510.068 21 5.139 0.674 1.6180 63.3 22 −2.515 0.228 1.8052 25.4 23 −3.3550.152 24 ∞ 3.185 1.5163 64.0 25 ∞ Magnification side power 126.1 times430.3 times Variable spacing 1 2.293 2.275 Variable spacing 2 4.6004.618

Values corresponding to Conditional expressions 1 to 4 (and Conditionalexpression 1′) in Example 3 are shown in Table 4, which will bedescribed below. As can be seen from Table 4, Example 3 satisfies all ofConditional expressions 1 to 4 (and Conditional expression 1′).

FIGS. 4 to 6 are diagrams illustrating all aberrations (sphericalaberration, astigmatism, distortion, and lateral chromatic aberration)of the projection lenses according to Examples 1 to 3. In the aberrationdiagrams, ω indicates a half angle of view. The spherical aberrationdiagram shows the aberration curves of the d-line, the F-line, and theC-line. The lateral chromatic aberration diagram shows the aberrationcurves of the F-line (represented by a dotted line, which is the samewith the other lateral chromatic aberration diagrams) and the C-line(represented by a two-dot chain line, which is the same with the otherlateral chromatic aberration diagrams) with respect to the d-line. Asshown in FIGS. 4 to 6, in the projection lenses according to Examples 1to 3, all aberrations including distortion and lateral chromaticaberration are effectively corrected, and the projection lenses have anF number of 2.04, which is a large value, and a total angle of view 2ωof 76.8 degrees, which is a wide angle.

The projection lens according to the invention is not limited to theabove-described examples, but various modifications and changes of theinvention can be made. For example, the curvature radius R of each lensand the on-axis surface spacing D between the lenses may beappropriately changed.

In addition, the projection display device according to the invention isnot limited to the above-mentioned structure, but the projection lensaccording to the invention may be applied to various apparatuses. Forexample, a transmissive or reflective liquid crystal display device or amicro mirror device having a plurality of tiltable micro mirrors formedon a substantial plane (for example, a digital micro mirror deviceavailable from Texas Instruments, Inc.) may be used as the light valve.In addition, the structure of the illumination optical system may beappropriately changed depending on the kind of light valves.

TABLE 4 (1)D_(G12)/f (2)f₁/f (3)f₂₃/f (4)n_(1G) Example 1 4.00 −6.783.92 1.83775 Example 2 5.01 −4.48 4.47 1.83775 Example 3 4.60 −5.75 4.001.83775

1. A projection lens consisting of: a first lens group having a negativerefractive power; a second lens group having a positive refractivepower; and a third lens group having a positive refractive power,wherein the first to third lens groups are arranged in this order from amagnification side, a lens unit arranged closest to a reduction side inthe first lens group is moved along an optical axis to adjust focus, andthe projection lens satisfies the following conditional expression:3.0<D _(G12) /f<6.0 where D_(G12) indicates a distance between the firstlens group and the second lens group and f indicates the focal length ofthe entire lens system.
 2. The projection lens according to claim 1,wherein the projection lens satisfies the following conditionalexpression:−8.0<f ₁ /f<−3.0 where f₁ indicates the focal length of the first lensgroup.
 3. The projection lens according to claim 1, wherein theprojection lens satisfies the following conditional expression:2.5<f ₂₃ /f<6.0 where f₂₃ indicates a composite focal length of thesecond lens group and the third lens group.
 4. The projection lensaccording to claim 1, wherein the lens unit arranged closest to thereduction side in the first lens group is a single positive lens havinga convex surface facing the reduction side.
 5. The projection lensaccording to claim 4, wherein the projection lens satisfies thefollowing conditional expression:1.8<n _(1G) where n_(1G) indicates the average value of the refractiveindexes of lenses in the first lens group with respect to the d-line,except for the lens unit arranged closest to the reduction side in thefirst lens group.
 6. The projection lens according to claim 5, wherein alens arranged closest to the reduction side in the third lens group is acemented lens of a positive lens and a negative lens.
 7. A projectiondisplay device comprising: at least one light source; at least one lightvalve; at least one illumination optical unit that guides light emittedfrom the light source to the light valve; and the projection lensaccording to claim 1, wherein the light valve modulates the lightemitted from the light source, and the modulated light is projected ontoa screen by the projection lens.